Improper Download Verification in Nextcloud Calendar App
CVE-2025-66550

5.7MEDIUM

Key Information:

Vendor

Nextcloud
Status
Security-advisories
Vendor
CVE Published:
5 December 2025

What is CVE-2025-66550?

The Nextcloud Calendar app, before version 4.7.17 and 5.2.4, is susceptible to a vulnerability allowing an attacker to create a calendar event with a maliciously crafted attachment. When a user attempts to access the event, the attachment is downloaded automatically from the same Nextcloud server without requiring user confirmation. This flaw poses a significant security risk as it can lead to unauthorized file downloads, potentially exposing sensitive data. Users are encouraged to upgrade to the latest versions to mitigate this issue.

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Deploy the Patch

Affected Version(s)

security-advisories >= 5.0.0-rc.1, < 5.2.4 < 5.0.0-rc.1, 5.2.4

security-advisories < 4.7.17 < 4.7.17

References

https://github.com/nextcloud/security-advisories/security...
x_refsource_CONFIRM
https://github.com/nextcloud/calendar/pull/6971
x_refsource_MISC
https://github.com/nextcloud/calendar/commit/63a6c398db01...
x_refsource_MISC

CVSS V3.1

Score:
5.7
Severity:
MEDIUM
Confidentiality:
None
Integrity:
High
Availability:
None
Attack Vector:
Network
Attack Complexity:
Low
Privileges Required:
Low
User Interaction:
Required
Scope:
Unchanged

Timeline

  • Vulnerability published

  • Vulnerability Reserved

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